Magnetic field evolution in magnetar crusts through three dimensional simulations

TL;DR

This study uses 3-D simulations to model magnetar crust magnetic field evolution, revealing how instabilities create intense small-scale features that explain diverse magnetar behaviors.

Contribution

It introduces detailed 3-D modeling of magnetar crust magnetic fields, highlighting the role of instabilities in forming energetic small-scale magnetic features.

Findings

Magnetic instabilities transfer energy to non-axisymmetric features.

Small-scale magnetic features can exceed global field strength.

These features can cause bursts and persistent emission.

Abstract

Current models of magnetars require extremely strong magnetic fields to explain their observed quiescent and bursting emission, implying that the field strength within the star's outer crust is orders of magnitude larger than the dipole component inferred from spin-down measurements. This presents a serious challenge to theories of magnetic field generation in a proto-neutron star. Here, we present detailed modelling of the evolution of the magnetic field in the crust of a neutron star through 3-D simulations. We find that, in the plausible scenario of equipartition of energy between global-scale poloidal and toroidal magnetic components, magnetic instabilities transfer energy to non-axisymmetric, kilometre-sized magnetic features, in which the local field strength can greatly exceed that of the global-scale field. These intense small-scale magnetic features can induce high energy bursts through local crust yielding, and the localised enhancement of Ohmic heating can power the star's persistent emission. Thus, the observed diversity in magnetar behaviour can be explained with mixed poloidal-toroidal fields of comparable energies.